Subsurface formation analysis has enabled more efficient oil and gas recovery over the past several decades. In recent years, petroleum exploration has been occurring at increasingly deeper levels of water. As the water levels increase and the wells drilled lengthen, the subsurface formations often become more complex. To facilitate more efficient petroleum recovery, it is often desirable to generate a vertical seismic profile.
A vertical seismic profile (VSP) is a class of borehole seismic measurements used for correlation between surface seismic receivers and wireline logging data. VSPs can be used to tie surface seismic data to well data, providing a useful tie to measured depths. Typically VSPs yield higher resolution data than surface seismic profiles provide. VSPs enable converting seismic data to zero-phase data as well as enable distinguishing primary reflections from multiples. In addition, a VSP is often used for analysis of portions of a formation ahead of the drill bit.
Narrowly defined, VSP refers to measurements made in a vertical wellbore using acoustic receivers inside the wellbore and a seismic source at the surface near the well. In a more general context as used herein, however, VSPs vary in well configuration, the number and location of sources and acoustic receivers, and how they are deployed. Nevertheless, VSP does connote the deployment of at least some receivers in the wellbore. Most VSPs use a surface seismic source, which is commonly a vibrator on land, or an air gun in marine environments.
There are various VSP configurations including zero-offset VSP, offset VSP, walkaway VSP, vertical incidence VSP, salt-proximity VSP, multi-offset VSP, and drill-noise or seismic-while-drilling VSP. Check-shot surveys are similar to VSP in that acoustic receivers are placed in the borehole and a surface source is used to generate an acoustic signal. However, a VSP is a more detailed than a check-shot survey. The VSP receivers are typically more closely spaced than those in a check-shot survey; check-shot surveys may include measurement intervals hundreds of meters apart. Further, a VSP uses the reflected energy contained in the recorded trace at each receiver position as well as the first direct path from source to receiver while the check-shot survey uses only the direct path travel time.
While VSPs can provide valuable information about a formation, source perturbations (e.g. shot to shot variations in the seismic signature of an air gun) introduce error into the raw seismic data which percolates through the processing chain to the final images produced. VSP source perturbations can limit the full range of usefulness that VSPs data can provide. In marine surface seismic acquisitions, these source perturbations can be well controlled through digital gun controllers and processes such as source signal estimation (see, for example, U.S. Pat. Nos. 4,757,482; 5,581,415; 5,995,905; and 4,476,553, which are hereby incorporated by reference).
However, in VSP acquisitions, particularly marine VSPs, there is currently no standard gun controller to limit error introduced by source perturbations. This lack of control is problematic, because the shot to shot variations in the source wavelet are often significant. These errors are caused by variations in the timing and firing pressure, which can be pronounced. In rough seas, elevation changes can also cause errors. Some have added an uncalibrated hydrophone near the source (usually located a few meters from the source) to provide partial information useful for correcting time break errors (errors attributable to time differences for high waves, irregular source firings, etc.). Nevertheless, the partial information from the added hydrophone is not sufficient for a full shot deconvolution because of the proximity of the source, and in practice, such hydrophones are arbitrarily placed in relation to the source and do not record with sufficient signal fidelity to be useful. As a result, only gross errors in VSP source data are currently corrected Therefore, sophisticated seismic data processing methods may not be used because current methods do not provide VSP seismic source information with the precision necessary to make sophisticated processing meaningful.
Further, some seismic systems include a cluster or array of air guns. Some of these systems fire all of the air guns simultaneously. The intent of the multiple air guns is to increase the amplitude of the seismic signal. However, the vertical spacing between the air guns and sea-surface results in an offset of the signals and what is sometimes referred to as “ghosting.”
Offset surveys come in a variety of options, ranging from the simple fixed offset survey to the complex Spiral 3D-VSP. Offset operations usually include a seismic source having one to twelve or more air guns, a remote installation (such as a boat) that moves according to a fixed offset, a walkaway line, or a spiral. There may be one or more boats involved, each with at least one seismic source. However, the most basic remote seismic measurement operations typically involve a single boat, at fixed position, firing one, two, or three air guns configured in a cluster.